Days away from Ireland's presidential election, the prospects of Martin McGuinness becoming the next incumbent are diminishing by the day. As the candidates head into the final straight before the vote on 27 October, Labour veteran Michael D Higgins is the frontrunner in the polls. Meanwhile, McGuinness's attempt to transfer a politically winning formula from north to south appears to have stalled.

Early in the campaign, the bookmaker Paddy Power was offering odds of 2-1 on for a victory for the deputy first minister of Northern Ireland, who has left that role for the duration of the campaign. Those odds have now lengthened to 20-1 against, as much of "middle Ireland" turns its back on the former IRA commander-turned-politician.

On Friday afternoon, outside Irish republicanism's most sacred spot – Dublin's General Post Office, where the 1916 Easter Rising began – McGuinness was confronted, not for the first time, by members of the public opposed to his politics.

On this occasion, the subject was not his record with the IRA, nor his alleged role in several high-profile atrocities. A cross-community group of environmental activists from Northern Ireland interrupted his canvass on O'Connell Street to object to Sinn Féin support for a new Dublin/Derry road link, which they say will mean the destruction of homes, farms and businesses. One of those who accosted McGuinness was Ciaran McClean, son of Paddy Joe McClean, a prominent activist in the Northern Ireland civil rights movement who was tortured by the British army in 1971.

"I ambushed McGuinness outside the GPO. I asked him why he supported taking half a billion euros out of the Irish economy for a road. His heavies [security guards] weren't too pleased," McClean said yesterday.

The turning point in the contest for McGuinness arguably came a fortnight ago, when a voter in Athlone confronted him with a photograph of Private Patrick Kelly, the Irish solider shot dead by the IRA in 1983 during an army attempt to rescue millionaire businessman Don Tidey, whom the Provos had abducted in an extortion bid.

In front of the cameras, David Kelly challenged McGuinness to break the IRA's code of omerta and give details of the men who killed his father. Kelly's intervention unnerved the normally cool, media-savvy McGuinness. It also raised the question of how someone who headed an organisation responsible for an Irish soldier's death could become commander-in-chief of the Irish army.

Kelly's televised clash with McGuinness prompted the families of other security force members in the Republic killed or wounded by the IRA to speak out. Last week, Sinn Féin cancelled a walkabout in Limerick city after Anne McCabe, the widow of murdered Garda Jerry McCabe, told the Limerick Leader that McGuinness was not fit to lead the Republic because he had held secret meetings with one of her husband's killers not long after the 1996 shooting.

According to Sinn Féin president Gerry Adams, it is time the southern Irish establishment got used to his party exercising and challenging for power. However, veteran IRA watchers including Ed Moloney, who wrote the definitive history of the Provisionals, A Secret History of the IRA, are not so sure. Moloney believes that by putting McGuinness into the presidential contest, the party has conjured up all the spectres of the IRA's recent past.

"In the north there is a tendency in the media not to put the actors in the peace process under too much scrutiny because the process couldn't withstand it," he said. "They forgot that in the south, the peace process marked the end of the need to indulge the north, so the media had no misgivings about putting McGuinness's very questionable account of his own life under scrutiny, and they found his cupboard as full of skeletons as is Gerry Adams's."

The campaign has undoubtedly been the most ferocious in modern Irish history. From Senator David Norris's alleged views on sex between men and boys, to allegations of sex abuse in the family of independent candidate and former Eurovision winner Dana, the tone has rarely been elevated.

Rising above it all has been Higgins. His closest rival remains the star of Ireland's version of Dragons' Den, Sean Gallagher, with McGuinness third in the running.

Joe Costello, the Irish Labour TD and election director, said yesterday that the man known affectionately as Michael D had benefited from staying out of the fray.

"Michael D said from the outset he would not get involved in personal attacks or recriminations. The people like that. They want a statesman, not someone flinging mud about."